Paying Change Fee Vs. New Reservation

I need to change the departure day for a USAirways flight to Montreal. The fare on the flight has dropped since I first booked. USAirways will charge $150 to change the reservation, but not lower the fare. If I just book a new outbound flight, it would cost less than $150. But if I don't use the first part of my original reservation, will it affect the return reservation?